Squelch circuits for radio receivers generally include a circuit to determine the signal to noise ratio of a signal derived by the radio receiver in response to the modulated waveform picked up by the receiver. In response to the signal to noise ratio being above a predetermined value, the detected modulated signal is coupled to an output device, such as a loud speaker. If the signal to noise ratio is so low that intelligent communication cannot be derived from the modulated carrier received, the detected modulated wave is not coupled to the output device so that an operator does not listen to a garbled, noise signal which would be annoying and unintelligible.
One of the three main categories that signal to noise ratio detectors generally utilized in prior art squelch circuits generally involves detecting the modulated carrier to derive an audio signal indicative of a voice or other audio signal modulated on the carrier. The audio signal is coupled to a spectral energy distribution network wherein the energy in two or more fixed frequency bands is compared to derive the squelch control signal. While this system has been widely adapted, it has certain disadvantages since the bandwidth of the signal coupled to the audio detector must be fixed; otherwise, sampling the spectral energy distribution in two more fixed frequency bands is not meaningful. A further disadvantage of responding to the detected audio signal is that such signal to noise ratio detecting circuits must be utilized in connection with a particular type of modulation detector. For example, if a receiver is adapted to be responsive to different types of AM and angle modulation having differing bandwidths, different audio response squelch networks must be employed for the different modulation types.
A second type of squelch control circuit has relied upon an AGC control voltage as an indication of the level of the signal to actuate a squelch controlled gate. The main problem with utilizing AGC as the basis for deriving a control voltage for a squelch gate is that the AGC voltage provides no indication as to the noise level on the signal. Instead, the AGC voltage merely indicates the strength of the signal; if the signal is weak, but is not noisy, the AGC level provides no indication of this fact. In the opposite manner, if the signal is strong, but there is a significant amount of noise on it, the AGC signal does not provide any indication of this fact. Because the AGC detector cannot discriminate between signal and noise, it is necessary to set a threshold level that is compared with the AGC voltage at a relatively high point so that the squelch gate is not triggered in response to high level noise alone. Typically, a minimum signal to noise ratio of approximately 20db is necessary for this type of squelch controller.
The most precise prior art signal to noise squelch control circuit of which I am aware includes a plurality of comb filters responsive to a received RF signal. A different portion of the received RF signal spectrum is supplied through each of the comb filters to a detector. Each detector indicates the presence of a signal component within the frequency band of the filter which derives the detector. The levels derived from the different detectors are compared with each other to derive an indication of signal to noise ratio over the entire RF band. Because of the requirement to utilize multiple narrow band bandpass filters, amplitude detectors and comparator circuits, the comb filter approach suffers from high cost, as well as great bulk and weight. It has the advantage, however, of not requiring prior knowledge of the type of modulation imposed on the RF carrier received by the radio receiver.